A memory device, such as non-volatile memory, is comprised of a plurality of memory cells. FIG. 1 is an example of a conventional memory cell 100. Memory cell 100 includes a control gate 102 in proximity to an oxide layer 104, and a floating gate 106. Fowler-Nordheim (FN) tunneling, also known as field emission, is a process used for programming memory cell 100. In FN tunneling, a high voltage level is applied to the control gate 102 via word line 108. A charge pump may be used to provide the high voltage levels to control gate 102, although any circuit that provides high voltage levels can be used, as desired.
Once the applied voltage level to control gate 102 passes a certain threshold voltage level, FN tunneling injection begins as current 113 flows from the bit line 110 through drain 112 to source 114. As current 113 flows through the floating gate 106, the drain 112 and source 114 are grounded. Concurrently, the floating gate 106 becomes negatively charged as electrons are injected and trapped in the layer. The negatively charged floating gate 106 may be interpreted as binary value 0 or 1 depending on the implementation, as desired. Since memory cell 100 is non-volatile, the cell retains its programmed state even when the applied voltage level is removed.
Memory cell reliability and the lifespan of a memory device is dependent on the high voltages and waveforms applied to the cell 100 during FN programming, in particular during FN tunneling injection. An attribute of an applied signal waveform is its slew rate. The slew rate is the maximum rate of change of a voltage signal, described by the relationship in Equation (1) as follows:
                    SlewRate        =                              max            ⁡                          (                                                ⅆ                  V                                                  ⅆ                  t                                            )                                .                                    Equation        ⁡                  (          1          )                    Therefore, the slew rate is the highest value derivative, or slope, of a signal waveform.
FIG. 2 illustrates an example of a conventional signal waveform 200 used for programming memory devices. From voltage level Vdd 204 to Vmax 208, signal 200 has a constant slew rate 202. The voltage level Vdd 204 is the power supply voltage. The voltage level Vmax 208 is a high voltage level required to change the state of memory cell 100 in FN programming. The voltage level Vthreshold 206 is a voltage level that triggers the initiation of FN tunneling injection and where long-term reliability of a memory cell may be compromised. The slew rate 202 of the prior art may result in unnecessary degradation of memory cells. Therefore, a need exits for improving memory cell reliability and lifespan.